Membranes made of cellulose acetate have been known for a long time. They exist in the form of flat membranes, tubular membranes, and hollow fiber membranes.
Thus, for example, DE-OS 20 59 048 describes a process for manufacturing a dry, semipermeable cellulose acetate membrane, composed of an active separating layer and a porous supporting layer. The membrane is dipped into an aqueous solution, containing in addition to a polyvalent alcohol with 2 or 3 carbon atoms in the molecule, at least one other substance, with the latter then being dried. The membrane is immersed in an aqueous solution containing as the other substance, 2 to 20 wt. % of an organic carboxylic acid with 1 to 3 carbon atoms in the molecule and 20 to 40 wt. % of the polyvalent alcohol with 2 to 3 carbon atoms in the molecule, and then being dried at a temperature between room temperature and 100.degree. C. The goal of this invention was to provide a membrane in which a treatment prior to drying prevents the separating action of the membrane not being adversely affected by drying.
DE-OS 26 19 250 describes a membrane one of whose surfaces is made by the dry phase inversion process while the other surface is made by the wet phase inversion process. These membranes are made by extruding a spinning solution containing a polymer in the form of a hollow fiber into an evaporation-promoting gaseous phase and injecting a nonsolvent for the polymer into the interior cavity of the hollow fiber, so that the exterior of the hollow fiber membrane is formed by the dry phase inversion process and the interior is formed by the wet phase inversion process. The result is dry-spun asymmetric hollow fibers.
DE-OS 26 06 244 describes a hollow fiber for membrane filtration consisting of a synthetic or semi-synthetic chain polymer that forms fibers during spinning and is characterized by its netlike structure. The hollow fiber is produced in the usual fashion by extruding the spinning solution through a spinneret, with a core-forming liquid for forming the interior cavity simultaneously being placed in the middle of the forming hollow fiber. The hollow fiber is spun downward with a fixed air gap.
In order to avoid the known problems that arise when using cavity-forming liquids during the manufacture of hollow fiber membranes, a variety of proposals have been made to use a gas instead of a liquid as the cavity-forming substance.
Thus, for example, EP 0 076 442 describes a hollow fiber made of regenerated copper ammonium cellulose, having a cylindrical bore on the lengthwise axis for the entire length of the fiber, with the length of the fiber being at least 10 m and the bore being filled with a gas.
This hollow fiber is obtained by extruding a spinning solution through an annular slot nozzle, with a gas being simultaneously introduced into the interior as it forms, and with the fiber being allowed to fall in free fall through an air gap and then dipping for a depth of up to 30 mm into a coagulation bath. Spinning solutions with high to very high viscosity are required for this purpose and are of course very difficult to handle.
According to the Derwent abstract of Japanese Application J 54027-025, hollow fiber membranes composed of polyvinyl alcohol are obtained when a spinning solution containing polyvinyl alcohol is spun through an annular slit opening, with a gas simultaneously being introduced into the interior cavity as it forms, in order to allow this internal cavity to coagulate. The coagulating gas can be ammonia or acetone vapor.
According to the Derwent abstract of Japanese Patent Application J 54055-623, hollow fiber membranes are obtained from acrylonitrile in which a spinning solution containing at least 50 wt. % acrylonitrile is spun through a nozzle and an inert gas or air is added simultaneously through the nozzle to form the interior cavity. The resultant membrane has a skin on its interior that is less than 0.5 .mu.m thick.
The above-mentioned membranes are only conditionally suitable for dialysis. In particular, these membranes leave much to be desired as far as their biocompatibility is concerned.
Membranes suitable for dialysis should be maximally biocompatible. A number of conditions must be met for this to be the case.
Substances that influence the biocompatibility of a membrane include albumin and .beta.2-microglobulin. .beta.2-microglobulin (molecular weight approximately 11,800) is loosely bonded to the surfaces of all cells containing nuclei as part of the main histocompatibility complex. This complex is responsible among other things for the compatibility of foreign tissue with the body's own tissue.
.beta.2-microglobulin is decomposed exclusively in the kidney and the daily production rate in a healthy individual is about 150 mg. Dialysis patients and uremics on the other hand have much higher .beta.2-microglobulin serum levels than healthy individuals. It is therefore very important for the .beta.2-microglobulin to be removed effectively during treatment.
The albumins likewise belong to the group of serum proteins, and constitute the largest group. The albumins maintain the colloidosmotic pressure and transport the body's own low-molecular substances as well as foreign ones. They also constitute the protein reservoir of the body.
Since the number of albumins is generally reduced in dialysis patients, it is important to ensure during treatment that albumin loss is kept as low as possible.
Depending on the area of application, a membrane must exhibit equally good performance parameters at different ultrafiltration rates, for example sieving coefficient.
In the past, however, it was usual for membranes that the corresponding screen coefficient for .beta.2-microglobulin and albumin at high ultrafiltration rates (high-flux range) did not reach these figures at average ultrafiltration rates (middle-flux range) or at low to very low ultrafiltration rates (low-flux range).
A membrane that functioned well in the low-flux range for example showed a sharp drop in terms of its separating effect in the high-flux range.